A tripeptide is formed by joining 3 amino acids in a sequence.

The question says:

• there are three different amino acids,
• each is used exactly once.

So the number of different tripeptides equals the number of different orders in which these 3 amino acids can be arranged.

If the amino acids are $A$, $B$, and $C$, possible sequences are:

• $ABC$
• $ACB$
• $BAC$
• $BCA$
• $CAB$
• $CBA$

Total number of arrangements of 3 distinct objects is: $3! = 3 \times 2 \times 1 = 6$

Therefore, the number of tripeptides formed is: $\boxed{6}$